1. Field of the Invention
This invention lies in the field of lasers and light emitting diodes (LEDs), and, more particularly of InGaAs and InGaAsP lasers and LEDs and methods for their manufacture.
2. Prior Art
Conventional InGaAsP/InP lasers and light emitting diodes (LEDs) suitable for use as continuous wave (CW) optical transmitters over the wavelength (.lambda.) range extending from about 1.2 to 1.5 microns (.mu.m) are constructed on an n-doped, (100)-oriented InP crystalline substrate. The layer structure is successively composed of an n-doped InP buffer layer, of an n-conductive or p-conductive active InGaAsP layer of the corresponding gap wavelength, and of a p-doped InP cover layer. Such cover layer, in most instances, is also followed by a p.sup.+ -doped InGaAs(P) contact layer in order to reduce the contact resistance. These lasers and LEDs are presently preferably produced by means of liquid phase epitaxy.
The optical fibers employed in optical communications technology in fact have their attenuation minimum in the wavelength (.lambda.) range extending from about 1.5 to 1.7 .mu.m. In order to produce lasers and LEDs which emit in this important wavelength range, it is necessary to correspondingly increase the gap wavelength .lambda..sub.g of the active InGaAsP layer. The highest gap wavelength .lambda..sub.g in the system InGaAsP is reached in the limit case In.sub.0.53 Ga.sub.0.47 As when .lambda..sub.g is about 1.65 .mu.m. The emission wavelength .lambda. of the ternary laser or of the ternary LED, lies in the range from about 1.65 through 1.70 .mu.m, depending especially on the doping, the injection current, and the temperature. Given the traditional layer build-up by means of liquid phase epitaxy (LPE), however, the following difficulty occurs when .lambda..sub.g is greater than or equal to 1.5 .mu.m: The active InGaAs(P) layer is partially melted back by the subsequent Jn-P solution that serves for the growth of the InP layer.
In order to avoid this melt-back effect, a quaternary anti-meltback layer having a gap wavelength below that of the active layer is grown between the active layer and the cover layer, as described by Y. Noguchi, K. Takahei, Y. Suzuki, and H. Nagai in the Japanese Journal of Appl. Phys., Vol. 19, No. 12, Dec. 1982, pages L 759 through L 762. This so-called anti-meltback layer, however, reduces the confinement factor of the waveguide and must therefore be selected so as to be extremely thin (about 0.1 .mu.m). This additional, extremely thin layer makes the LPE process more complicated, and, thus, less reproducible.
S. H. Groves and M. C. Plonko (Appl. Phys. Letters, Vol. 38, No. 12, June 1981, pages 1003 through 1004) have shown that meltback-free growth of InP on (100)-oriented In.sub.0.53 Ga.sub.0.47 As (and, thus, on InGaAsP as well) is possible from an Sn-In-P solution. The InP layer thus produced, however, is highly n-doped. It is therefore unsuitable for use as a cover layer for the known laser and LED structures based on n-InP substrates, since a p-doped InP cover layer is thereby required.
It is also known that the structuring of the epitaxial layer sequence in the form of a so-called "mushroom" structure can be carried out in order to produce an index-guided laser with a low threshold current and a high optical output power, as described by H. Burkhard and E. Kuphal in the Japanese Journal of Appl. Phys., Vol. 22, No. 11, Nov. 1983, pages L 721 through L 723. The "mushroom" structure lasers have electrical and optical properties which are equivalent to the usually produced so-called "buried hetero-structure lasers" (BH-lasers), but have the advantage of a far simpler technology, since only one epitaxial process is required in contrast to the two epitaxial processes required for BH-lasers.